The invention relates to seafloor electromagnetic surveying for oil and other hydrocarbon reserves.
Determining the response of subterranean strata within the earth's crust to electromagnetic fields is a valuable tool in the field of geophysical research. The geological structures associated with thermally, hydrothermally, tectonically or magmatically active regions can be studied. In addition, electromagnetic surveying, or sounding, techniques can provide valuable insights into the nature, and particularly the likely hydrocarbon content, of subterranean reservoirs in the context of subterranean oil exploration and surveying.
Seismic techniques are often used during oil exploration to identify the existence, location and extent of reservoirs in subterranean rock strata. Whilst seismic surveying is able to identify such structures, the technique is often unable to distinguish between the different possible compositions of pore fluids within them, especially for pore fluids which have similar mechanical properties. In the field of oil exploration, it is necessary to determine whether a previously identified reservoir contains hydrocarbons or just aqueous pore fluids. To do this, an exploratory well is drilled to determine the contents of the reservoir. However, this is an expensive process, and one which provides no guarantee of reward.
Whilst hydrocarbon-filled and water-filled reservoirs are mechanically similar, they do possess significantly different electrical properties and these provide for the possibility of electromagnetic based discrimination testing.
A known technique for electromagnetic probing of subterranean rock strata is the passive magneto-telluric (MT) method. Signals measured by a surface-based electromagnetic receiver in response to electromagnetic (EM) fields generated naturally, such as within the earth's upper atmosphere, can provide details about the surrounding subterranean rock strata. However, MT fields excite predominantly horizontal current flows in the earth and this makes MT surveying intrinsically insensitive to the thin resistive layers typical of subterranean hydrocarbon reservoirs. Furthermore, MT data are rarely collected at the seafloor at frequencies high enough to resolve subterranean strata on scales typical of hydrocarbon reservoirs. In addition, the effect of distant coastlines can also often be seen in MT data. This increases the complexity of data interpretation. Notwithstanding these limitations, MT techniques are still useful for determining large-scale background structure in a subterranean strata configuration, even if they cannot be directly applied to surveying for subterranean hydrocarbon reservoirs [7].
Because of the different electrical properties of hydrocarbon-filled and water-filled reservoirs, measurements of electrical resistivity beneath the seafloor have traditionally played a crucial role in hydrocarbon exploration and reservoir assessment and development. In industry, subterranean resistivity data have generally been obtained almost exclusively by wire-line logging of wells. There are, though, clear advantages to developing non-invasive geophysical methods capable of providing such information from the surface or seafloor. Although such methods are unlikely to provide comparable vertical resolution to wireline logging, the vast saving in terms of avoiding the costs of drilling test wells into structures that do not contain economically recoverable amounts of hydrocarbon would represent a major economic advantage.
In research fields that are not of commercial interest, geophysical methods for mapping subterranean resistivity variations by various forms of controlled source EM surveying have been in use for many years [1, 2, 3, 10]. Proposals for finding hydrocarbon reservoirs using such EM surveying techniques have also been made [4, 5], and applications to the direct detection of hydrocarbons using horizontal electric dipole (HED) transmitters (or sources) and receivers (or detectors) have proved successful [6, 7].
FIG. 1 schematically shows a surface vessel 14 undertaking controlled source EM surveying of a subterranean strata configuration according to a previously proposed method [6]. The subterranean strata configuration includes an overburden layer 8, an underburden layer 9 and a hydrocarbon reservoir 12. The surface vessel 14 floats on the surface 2 of a body of water, in this case seawater 4 of depth d meters. A submersible vehicle 19 carrying a HED transmitter 22 is attached to the surface vessel 14 by an umbilical cable 16 providing an electrical and mechanical connection between the submersible vehicle 19 and the surface vessel 14. The HED transmitter is supplied with a drive current so that it broadcasts a HED EM signal into the seawater 4.
One or more remote receivers 25 are located on the seafloor 6. The receivers 25 include an instrument package 26, an antenna 24, a floatation device 28 and a ballast weight (not shown). The antenna 24 comprises an orthogonal pair of horizontal electric dipole detectors. The horizontal electric dipole detectors are sensitive to EM fields induced by the HED transmitter in the vicinity of the receiver 25, and produce detector signals therefrom. The instrument package 26 records the detector signals for later analysis.
The HED transmitter 22 broadcasts EM signals that propagate outwards both into the overlying water column 4 and downwards into the seafloor 6 and the underlying strata 8, 9, 12. At practical frequencies for this method and given the typical resistivity of the respective media 4, 8, 9, 12, propagation occurs by diffusion of electromagnetic fields. The rate of decay in amplitude and the phase shift of the signal are controlled both by geometric spreading and by skin depth effects. Because in general the underlying strata 8, 9, 12 are more resistive than the seawater 4, skin depths in the underlying strata 8, 9, 12 are longer. As a result, electric fields measured by a receiver located at a suitable horizontal separation are dominated by those components of the transmitter EM signal which have propagated downwards through the seafloor 6, along within the underlying strata 8, 9, 12, and back up to the receiving antenna 24. Both the amplitude and the phase of the received signal depend on the resistivity structure of the underlying strata 8, 9, 12. Accordingly, a survey built up from many transmitter and receiver locations can provide a multi-dimensional image, by geophysical inversion, of subterranean resistivity. Because hydrocarbon reservoirs have relatively high resistivities (typically 100 Ωm) compared to other subterranean strata (typically 1 Ωm), they can be easily identified in maps of subterranean resistivity.
However, a significant problem with controlled source EM surveying techniques of the kind shown in FIG. 1 is that they do not work well in shallow water due to the presence of an ‘air-wave’ component in the EM fields induced by the HED transmitter at the receiver. This air-wave component is due to EM signals from the HED transmitter which follow a propagation path upwards through the seawater to the surface; horizontally through the air; and back down through the water column to the receiver. The air-wave component contains very little information about subterranean resistivity. Accordingly, if the air-wave contributes a significant component to the EM fields induced by the HED transmitter at the receiver, the sensitivity of the technique to subterranean resistivity structures, such as hydrocarbon reservoirs, is greatly reduced. The path of an example air-wave component is shown in FIG. 1 by a dotted line labelled AW. The magnitude of the air-wave component is not significantly reduced by its passage through air. This is because air is non-conducting. However, as with other components, the airwave component is strongly attenuated by its passage through the seawater. This means that in relatively deep water (large d) the air-wave component is not very significant at the receiver and as such does not present a major problem. However in shallow water (small d) the air-wave component does not pass through as much seawater and thus makes a larger contribution to the EM fields induced by the HED transmitter at the receiver. This contribution becomes greater still at increasing transmitter-receiver horizontal separations. This is because (other than due to geometric spreading) the strength of the air-wave component is relatively constant over a wide range of horizontal separations since any extra distance travelled by the air-wave component is almost exclusively in the non-attenuating air. Other components of the EM fields induced by the HED at the receiver, such as those which pass through the subterranean strata and are of interest, travel through lower resistivity media and become increasing attenuated as they travel further. For these reasons, the air-wave component tends to dominate the EM fields induced by the HED transmitter at the receiver for surveys made in shallow water, especially at long transmitter-receiver horizontal separations.
The existence of the air-wave as a dominant component of the detector signals limits the applicability of the above described surveying technique. In shallow water the range of transmitter-receiver over which the technique can be applied is much reduced. This not only leads to a need to employ more receiver locations to adequately cover a given area, but also limits the depth beneath the seafloor to which the technique is sensitive. This can mean that a buried hydrocarbon reservoir in shallow water may not be detectable, even though the same reservoir would be detected in deeper water.
FIG. 2 is a graph schematically showing results of one-dimensional modelling of two example EM surveys of the kind shown in FIG. 1. One example corresponds to a survey performed in deep water (dotted line) and the other to a survey performed in shallow water (solid line). For each model survey the amplitude of an electric field component induced at the receiver in response to the HED EM transmitter is calculated per unit transmitter dipole and is plotted as a function of horizontal separation R between the HED transmitter and the receiver. For both model surveys, the subterranean strata configuration is a semi-infinite homogeneous half space of resistivity 1 Ωm. In the deep-water example, the subterranean strata configuration is located beneath an infinite extent of seawater. In the shallow-water example, it is located beneath a 500-meter depth of seawater. In both cases the seawater has resistivity 0.3 Ωm. The transmitter and receiver are separated along a line which runs through the axis of the HED transmitter. It is the component of detected electric field resolved along this direction which is plotted in FIG. 2. The HED transmitter is driven by a quasi-square wave AC current at a frequency of 0.25 Hz.
The effect of the air-wave component on the amplitude of EM fields induced by the HED transmitter at the receiver is clear. In the deep-water model survey, where there is no air-wave component, the calculated electric field amplitude falls steadily with increasing horizontal separation. In the shallow-water model, however, where there is a strong air-wave component, the rate of amplitude reduction sharply reduces at a transmitter-receiver horizontal separation of about 5000 m. FIG. 3 is a plot showing the ratio, p, of the two curves shown in FIG. 2, and the large deviations from unity highlight the difference between these curves. Since the only difference between the two model surveys is the presence or not of an air-wave component, the ratio plotted in FIG. 3 effectively shows the relative strength of the air-wave component in the detected signal compared to that which passes through the subterranean strata for the shallow-water model survey.
It is apparent from FIGS. 2 and 3 that at all but the very shortest horizontal separations the detected electric field is significantly larger in the shallow-water model. For example, at a horizontal separation of 2500 m, the amplitude of the detected signal in the deep-water model survey is around 10−12 Am−2. In the shallow-water model survey it is higher at around 10−11.5 Am−2. This is due to the additional contributions of the air-wave component. This level of increase shows that the air-wave component is over twice as strong as the component which has passed through the subterranean strata, and accordingly over two-thirds of the detector signal carries almost no information about the subterranean strata. At greater horizontal separations the air-wave component dominates even more. In particular, it becomes especially pronounced beyond around 5000 m. At this point there is a break in the rate at which the detected electric field amplitude falls with increasing horizontal separation. At a horizontal separation of around 7000 m, the air-wave component in the shallow water example is around twenty times stronger than that which passes through the subterranean strata. This clearly imposes high requirements for the signal-to-noise ratio of data collected over these sorts of horizontal separations, as is generally the case when a small signal rides on a large background. It is apparent that the air-wave significantly limits the usefulness of these surveying techniques in shallow water.
In addition to the problems associated with the air-wave component, practical EM surveys of the kind shown in FIG. 1 are subject to several other limitations. These limitations arise because of the need to carefully consider the orientation of the current flows induced by EM signals [6]. The response of seawater and subterranean strata (which will typically comprise planar horizontal layers) to EM signals is generally very different for horizontally and vertically flowing current components. For horizontally flowing current components, the coupling between the layers comprising the subterranean strata is largely inductive. This means the presence of thin resistive layers (which are indicative of hydrocarbon reservoirs) do not significantly affect the EM fields detected at the surface since the large scale current flow pattern is not affected by the thin layer. On the other hand, for vertical current flow components, the coupling between layers is largely galvanic (i.e. due to the direct transfer of charge). In these cases even a thin resistive layer strongly affects the EM fields detected at the surface since the large scale current flow pattern is interrupted by the resistive layer. It is known therefore that vertical current loop components of induced current are required to satisfactorily perform an EM survey in the field of oil exploration.
However, sole reliance on the sensitivity of vertical current flow components to the presence of a thin resistive layer can lead to ambiguities. The effects on detected EM fields arising from the presence a thin resistive layer can be indistinguishable from the effects arising from other realistic large scale subterranean strata configurations. In order to resolve these ambiguities using known survey techniques it is necessary to determine the response of the subterranean strata to both horizontal (i.e. inductively coupled) and vertical (i.e. galvanically coupled) induced current flows [6].
Hence it is important when designing a practical EM survey of the kind described above to distinguish between transmitter and receiver configurations in which the coupling between layers is largely inductive due to horizontal currents (in which case the survey has little sensitivity to the presence of a thin reservoir) and those in which the coupling between layers is largely galvanic due to vertical currents (in which case blocking of the passage of this current flow by a reservoir leads to a survey which is strongly sensitive to the presence and boundary of hydrocarbon within the reservoir).
The HED transmitter 22 shown in FIG. 1 generates both inductive and galvanic current flow modes with the relative strength of each mode depending on HED transmitter-receiver geometry. At receiver locations which are broadside to the HED transmitter axis, the inductive mode dominates the response. At receiver locations which are in-line with the HED transmitter axis, the galvanic mode is stronger [6, 8, 9, 10]. The response at receiver locations in both the in-line and broadside configurations is governed by a combination of the inductively and galvanically coupled modes and these tend to work in opposition.
FIG. 4 shows in plan view an example survey geometry according to the above described survey method in which sixteen receivers 25 are laid out in a square grid on a section of seafloor 6 above a subterranean reservoir 56 having a boundary indicated by a heavy line 58. The orientation of the subterranean reservoir is indicated by the cardinal compass points (marked N, E, S and W for North, East, South and West respectively) indicated in the upper right of the figure. To perform a survey, a transmitter starts from location ‘A’ and is towed along a path indicated by the broken line 60 through location ‘B’ until it reaches location ‘C’ which marks the end of the survey path. As is evident, the tow path first covers four parallel paths aligned with the North-South direction to drive over the four “columns” of the receivers. This part of the survey path moves from location ‘A’ to ‘B’. Starting from location ‘B’, the survey path then covers four paths aligned with the East-West direction which drive over the four “rows” of receivers. Each receiver is thus driven over in two orthogonal directions. The survey is completed when the transmitter reaches the location marked ‘C’.
During the towing process, each of the receivers 25 presents several different orientation geometries with respect to the transmitter. For example, when the transmitter is directly above the receiver position D1 and on the North-South aligned section of the tow path, the receivers at positions D5, D6 and D7 are at different ranges in an end-on position, the receivers at positions D2, D3 and D4 are at different horizontal separations in a broadside position and the receiver at positions D8 and D9 are in-between. However, when the transmitter later passes over the receiver position D1 when on the East-West aligned section of the tow path, the receivers at positions D5, D6 and D7 are now in a broadside position, and the receivers at position D2, D3 and D4 are in an end-on position. Thus, in the course of a survey, and in conjunction with the positional information of the transmitter, data from the receivers can be used to provide details of the signal transmission through the subterranean strata for a comprehensive range of distances and orientations between transmitter and receiver, each with varying galvanic and inductive contributions to the signal propagation. In this way a simple continuous towing of the transmitter can provide a detailed survey which covers the extent of the subterranean reservoir.
While this survey method has been demonstrated to provide good results in practice, as noted above some limitations have been identified.
Firstly, since the inductive and galvanic modes cannot be easily separated there will generally be a level of cross-talk between them at a receiver. This may lead to ambiguities in the results.
Secondly, in order to obtain survey data from both in-line and broadside geometries, the HED transmitter needs to be re-oriented at each HED transmitter survey location. This requires the surface vessel to make multiple passes over broadcast locations and can lead to complex and long tow patterns.
Thirdly, a HED transmitter based EM survey can only provide the best data possible at discrete survey locations. This is because of the geometric requirements of a HED transmitter survey which dictate that, at any point during the survey, data can only be optimally collected from those receivers to which the HED transmitter is arranged either in-line or broadside. At other orientations, horizontal separation of the inductively and galvanically coupled signals becomes much more difficult and data are less reliable. For instance, referring to FIG. 4, when the HED transmitter is at a point on the tow path above the receiver marked D1 and on the North-South aligned section of the tow path, in-line data can only be collected from the receivers marked D5, D6 and D7, whilst broadside data can only be collected form the receivers marked D2, D3 and D4. The other receivers provide only marginally useful information at this point of the survey. Furthermore, if the HED transmitter is at the location identified by reference numeral 57 in FIG. 4, which is on a North-South aligned section of the tow path, in-line data can be collected from the receivers marked D3, D8, D9 and D10, but broadside data cannot be collected from any of the receivers. Since both broadside and in-line data are required for optimal analysis, the best data possible with the square receiver array shown in FIG. 4 can only be collected from points along the tow path directly above the receiver locations.